The invention concerns a magnet system that generates a highly stable magnetic field at a sample location, the magnet system comprising a magnet cryostat housing a first superconducting magnet coil and a second magnet coil co-axially to said first magnet coil which second magnet coil is during operation of the magnet system short-circuited in a superconducting persistent mode.
Such a magnet system in known from U.S. Pat. No. 4,974,113.
The innermost sections of magnet coils producing very high fields may be HTS insert coils that are separately driven by a separate power supply. Outer LTS coils may be operated in a common persistent mode or alternatively also during operation be connected to a power supply which may be the same one. Use of a separate power supply for the HTS insert coils has inter alia the advantage that the operating current of the HTS coils can be different from that of the LTS coils and that the non-persistent HTS current circuit need not be perfectly without any losses since the current is determined by the power supply. However, the power supply introduces noise and instabilities into the system with the consequence that such a system will be unsuitable for magnetic resonance applications which at the sample location need a highly stable, drift-free magnetic field.
From U.S. Pat. No. 4,974,113 a magnet system is known that uses a superconducting main magnet coil and with a superconducting compensation coil in the form of a superconducting closed current path that is inductively coupled to the main magnet coil. External perturbations may cause oscillations of a magnetic field generated by the first coil. These oscillations may be compensated by the compensation coil which can inductively react to changes of the magnetic flux through its cross-section. U.S. Pat. No. 6,307,370 B1 discloses a significantly more complex system, that (as a function of time or frequency, respectively) can also take into account the presence of resistive current circuits (e.g. protection resistors).
NMR magnets for highest magnetic fields as a rule consist of several nested, largely solenoid-shaped superconducting partial coils which are jointly operated in series in persistent current mode.
Presently, pure NMR-suitable magnet coil systems that operate in superconducting persistent current mode are available up to 23.5 Tesla, i.e. 1000 MHz proton frequency. Using presently available LTS materials, the field strength cannot be noticeably increased further since in that case the critical superconductive current will be exceeded in the magnetic field. In order to reach proton frequencies, e.g. around 1200 MHz, in a stable and homogeneous way, in the present situation, the innermost sections of the magnet coil systems must be made from HTS conductors.
However, up to now there are no superconducting joints for HTS conductors, in particular for their connection to LTS conductors like NbTi, which during operation with full design current of a magnet coil with windings inter alia made from these HTS conductors would reliably have sufficiently low ohmic losses in order to enable a highly stable operation in superconducting persistent mode with a sufficiently low field drift that is e.g. necessary for an NMR spectrometer of highest resolution. Moreover, for HTS conductors, the quality of the conductor itself may pose a problem regarding the necessary large conductor lengths with the consequence that also defects within the conductor may prevent persistent operation with sufficient low drift.
Therefore it is the object of the invention to improve a magnet system with a coil having a drifting superconductor or a superconductor with joints that introduce a drift that cannot be compensated by state of the art drift compensation measures, in a way that with such a magnet system a highly stable magnetic field can be generated.